Impact of water masses on plankton community respiration in the southern East China Sea

The southern East China Sea (ECS) is influenced by four major water masses—China Coastal Water, Taiwan Warm Current (TWC), upwelling water, and Kuroshio Water—each characterized by distinct hydrographic and biogeochemical properties. Our results show pronounced spatial contrasts in nutrient availability and planktonic processes among these water masses. Nitrate concentrations were highest in China Coastal Water, followed by upwelling water and TWC, and lowest in Kuroshio Water. Correspondingly, chlorophyll a and particulate organic carbon (POC) exhibited decreasing gradients from coastal to offshore waters. Bacterial production and plankton community respiration (CR) closely mirrored these patterns, with the highest rates observed in China Coastal Water, intermediate values in upwelling and TWC regions, and minimal rates in Kuroshio Water. Mean plankton CR integrated over the upper 40 m ranged from ~129 mg C m⁻³ d⁻¹ in coastal waters to ~11 mg C m⁻³ d⁻¹ in Kuroshio waters. Significant positive relationships were identified between plankton CR and chlorophyll a, POC, and bacterial production, indicating tight coupling among phytoplankton biomass, microbial activity, and organic carbon consumption. These findings highlight the dominant role of nutrient-rich coastal and upwelling waters in driving organic carbon remineralization on the southern ECS shelf and underscore the importance of water-mass mixing in regulating carbon cycling and ecosystem functioning across this dynamic marginal sea.